Malfunction Indicator Lamp (MIL) Always On

SYSTEM DESCRIPTION
The Diagnostic System Check-Computers and Controls Systems must be the starting point for any driveability complaint diagnosis. Perform a careful inspection of the powertrain control module (PCM) and engine grounds for being clean and tight. Refer to Visual and Physical Check in Symptoms - Computers and Controls Systems. Diagnostic System Check - Computers and Controls Systems is an organized approach to identifying a problem created by an electronic engine control system malfunction. Initial Inspection and Diagnostic Overview

The powertrain control module (PCM) controls the malfunction indicator lamp (MIL) by providing a ground path through the MIL control circuit to turn ON the MIL. When the ignition is turned ON, the MIL will remain on until the engine is running, if no diagnostic trouble codes (DTC) are stored. If the MIL is illuminated, then the engine stalls, the MIL will remain illuminated so long as the ignition is ON.

MIL OPERATION
The MIL is located on the instrument panel and is displayed as CHECK ENGINE lamp.

MIL FUNCTION
^ The MIL informs the driver that a malfunction has occurred and the vehicle should be taken in for service as soon as possible.
^ The MIL illuminates during a bulb test and a system test.
^ A DTC will be stored if a MIL is requested by the diagnostic.

MIL ILLUMINATION
^ The MIL will illuminate with ignition ON and the engine not running.
^ The MIL will turn OFF when the engine is started.
^ The MIL will remain ON if the self-diagnostic system has detected a malfunction.
^ The MIL may turn OFF if the malfunction is not present.
^ If the MIL is illuminated and then the engine stalls, the MIL will remain illuminated so long as the ignition switch is ON.
^ If the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition switch is cycled OFF, then ON.

DIAGNOSTIC SYSTEM CHECK
Perform the Computers and Controls Systems Diagnostic System Check, when the following conditions are present:
^ When the MIL does not turn ON when the ignition switch is turned to the RUN position. See above for MIL operation.
^ When the MIL remains ON while the engine is running
^ When the MIL is flashing while the engine is running
^ When a driveability symptom is determined Initial Inspection and Diagnostic Overview

TEST DESCRIPTION

Steps 1-7:

The number below refers to the step number on the diagnostic table.
5. Using a test lamp connected to battery positive voltage, probe the MIL control circuit at the PCM.